You do have a bit of a busy Pi-hole, but nothing that should cause memory or other problems. When your internet stops again, please run the following commands from the command prompt or terminal of a connected client (and not from the Pi terminal):
nslookup pi.hole
nslookup flurry.com 192.168.1.42
So just from my desktop, rather than from the SSH client?
Yes, from the desktop on a client (not the Pi desktop, if you have one) and not via ssh session to the Pi terminal. You want to run these commands directly on the client OS.
I don't want to leave this hanging, but it seems to have gone away? There hasn't been an outage since the last message, which seems strange, but there it is. I'm certainly worried that this will return, but at the moment there's no issue.
Well, it just happened again. I unfortunately didn't have this tab open, but will reconfigure the DNS settings back to the pihole after class and try again.
A couple of things can stop/crash pihole-FTL:
On Raspberry Pi's, under-voltage can ocure:
503 Service Not Available - #26 by deHakkelaar
Running out of memory.
Maybe bc dbase grows too large bc of DNS loop or other excessive querying:
DNS not running, FTL down - #24 by deHakkelaar
Filesystem errors:
Pihole intermittently stops resolving until hardware reboot - #14 by deHakkelaar
Or maybe a power saver if connected via WiFi:
And I'm probably missing a few others 
Ok! It just happened again. Results as below:
C:\Users\jbrat>nslookup pi.hole
DNS request timed out.
timeout was 2 seconds.
Server: UnKnown
Address: 192.168.1.42
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
*** Request to UnKnown timed-out
C:\Users\jbrat>nslookup flurry.com 192.168.1.42
DNS request timed out.
timeout was 2 seconds.
Server: UnKnown
Address: 192.168.1.42
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
Name: flurry.com
Address: 0.0.0.0
No. Appears to be an intermittent stall of some sort. The first nslookup completely timed out, but the second one correctly provided the answer to the query.
I would run pihole -r and select repair. May not fix it, but can't hurt anything either.
Well, it didn't help anything, but it doesn't seem to have hurt! Is there anything else I can try, or am I SOL?
I see you have very active clients, like
Is this your router? If so, why does it query > 180.000 domains in 24 hours? Is there a DNS loop? Your device may be stale when trying to store this massive amount of queries onto your SD card if it is slow.
Oh dang! Yeah, I had no idea what that meant! How do I resolve that?
Well, depends on your router and what you see. When you navigate to your dashboard -> Query Log, I assume the vast majority of queries comes from your router. What is the domain they query?
And how did you configure the Pi-hole in your router? Did you use the router then again as upstream destination in Pi-hole?
I'm not seeing any identifiable pattern, although I don't understand why there are so many background queries. They're not coming from my router though - I've got the pi-hole passing out ip's, so most of the traffic I'm seeing comes from my computers, with a few queries from my smart home devices.
edit: Ok, the largest number is coming from an LG device, but not my TV. To the best of my knowledge, there are only 3 LG devices in the house, which means either the laundry machine or dryer is putting out a ton of queries (750,000)
edit 2: More the fool I: That LG device is actually my phone.
Sorted by number of queries:
The early time-outs you see in your nslookup may indeed indicate a DNS loop, limited to local lookups.
Both nslookup prompt time-outs when trying to resolve local names (pi-hole) or IP addresses (192.168.1.42).
This may be the case when you enable Conditional Fowarding (CF) while having your router use Pi-hole as least as one of its upstream DNS servers.
Try if either disabling CF in Pi-hole or avoiding Pi-hole as upstream DNS in your router would resolve your issue.
If you don't know that, you may well not be using those features, ruining my theory for fixing your issue altogether. 
You'll find Conditional Fowarding to the very bottom of Pi-hole's Settings | DNS pane.
As to how and whether you'd configure your router to use Pi-hole as upstream DNS:
I honestly wouldn't (and couldn't) have a clue, since I do not know your router.
In general, a router may allow you to either configure upstream DNS (commonly, a WAN or Internet setting) or local DNS servers (a LAN / DHCP setting).
If you can configure Pi-hole as the latter, there would be less or no need to configure it as the former.
It looks like CF is off, and always has been. I am using the pihole as the DHCP server, so maybe I configured that wrong in some way?
What's your Pi-hole using as upstream DNS servers?
